Soft-delete DELETE /api/user/ for authenticated users (hide conversations, blacklist tokens, block staff self-delete). Sync Stripe portal cancel/change via subscription.updated/deleted webhooks and expose cancel_at_period_end for Account UI (chat_web_app#75 companion).
## Summary Implements [#16](#16), [#17](#17), and [#36](#36) in one backend PR. - **#36 Multi-plan catalog**: Founders ($10, public), Standard ($15), Pro ($40), Business ($99), Backer ($0). Future tiers seeded but hidden/`is_selectable=false`. Backer email whitelist auto-assigns Founders-level access with no checkout. - **#36 Feature + prompt gating**: plan feature flags (text vs image); rolling **6h** prompt windows (100 / 200 / 300 / 300 / 300). Enforced in both chat consumers when `ENFORCE_SUBSCRIPTION_GATES=true`. - **#17 Token-period quotas**: optional `monthly_token_quota` on plans + per-user override; calendar-month aggregation from `PromptMetric`; warn/block when reported token totals exceed cap. Null provider usage never fabricated as 0; tracked via `turns_missing_token_usage`. - **#16 Token API exposure**: `tokens_in` / `tokens_out` on conversation + prompt serializers (null when unknown). `GET /api/finance/subscription/` returns plan + usage snapshot for the FE. - Checkout defaults to **Founders**; Stripe paid webhooks assign Founders. Registration/OAuth redeem Backer whitelist and return `needs_checkout`. Companion FE PR: `chat_web_app` branch `feature/plans-quotas-token-usage`. ## Test plan - [ ] `manage.py migrate` seeds five plans; admin can add Backer emails - [ ] Public `GET /api/finance/plans/` returns only Founders - [ ] Register with Backer email → active Backer, `needs_checkout=false`, checkout rejected - [ ] Founders checkout + paid webhook → active Founders subscription - [ ] Chat turn blocked without subscription / when prompt window exceeded / when token period exceeded - [ ] Standard plan denies image feature; Pro/Founders/Backer allow - [ ] Conversation/prompt API returns `null` tokens when unreported, sums when present - [ ] `finance.tests.test_plans_quotas` + existing finance/checkout tests passReviewed-on: #37